1. Field of the Invention
This invention relates to a gear-shifting control system for an automatic transmission, and more particularly to a gear-shifting control system for an automatic transmission in which gear-shifting is controlled according to a backup gear-shifting characteristics for emergency use in case of failure of a sensor for detecting the output shaft rpm of the torque converter.
2. Description of the Prior Art
There has been in wide use an automatic transmission having a torque converter in combination with a multiple stage transmission gear mechanism such as a planetary gear-train. Generally, such an automatic transmission is controlled by way of a hydraulic mechanism in which fluid actuators such as brakes and clutches incorporated in the multiple stage transmission gear mechanism are controlled to change the transmission systems of the engine output power to obtain a desired transmission speed by the switching of hydraulic circuits using electromagnetic type switching valves. The running condition of a vehicle is detected by an electronic control device and the hydraulic circuits are switched to effect gear-shifting when the running condition of the engine crosses a predetermined gear-shifting line. Generally the hydraulic circuits are switched by selectively operating the electromagnetic type switching valves according to an upshifting signal or a downshifting signal from the electronic control device.
In controlling the automatic transmission, the torque converter output shaft rpm, i.e., the turbine rpm, functions as a parameter for determining the gear-shifting characteristics and is a very important factor. There has been known an automatic transmission in which lockup is released when the part for detecting the turbine rpm fails (see Japanese Unexamined Patent Publication No. 57(1982)65454). However, if failure of the part for detecting the turbine rpm is dealt with by simply releasing lockup, there is a risk that gear-shifting will not be effected or that the automatic transmission will shift to the wrong gear, e.g., the transmission with downshift to a very low speed gear during a high speed travel, rendering further travel impossible.